SE2051028A1 - A method for producing a machine glazed paper comprising microfibrillated cellulose and a machine glazed paper - Google Patents

Abstract

The present invention relates to a method for producing a machine glazed paper comprising microfibrillated cellulose, wherein the method comprises the steps of: providing a suspension comprising between 0.1 wt-% to 50 wt-% of microfibrillated cellulose based on total dry weight, forming a fibrous web of said suspension on a wire wherein said web has a dry content of 1-25% by weight, dewatering the fibrous web in at least one dewatering unit, glazing at least one side of the dewatered fibrous web in a glazing unit to form the machine glazed paper. The invention further relates to a MG paper produced according to the method.

Description

A METHOD FOR PRODUCING A MACHINE GLAZED PAPERCOMPRISING MICROFIBRILLATED CELLULOSE AND A MACHINEGLAZED PAPER Technical fieldThe present invention relates to a method for producing a machineglazed paper comprising microfibrillated cellulose and a machine glazed paper comprising microfibrillated cellulose produced according to the method.

Background Machine glazed (MG) paper is a paper used in label paper, specialprinting application and in different food and hygiene packaging applications.Normally, one surface of the paper is glazed, i.e. treated in such a way thatthe gloss of the surface of the paper is increased. The glazing of the at leastone surface of the paper is done in order to provide the paper with improvedgloss and increased surface density without losing too much bulk. The glazedsurface improves the barrier properties, especially improved barrier againstgrease and oil as well as it gives the surface improved printing properties.

Besides having good barrier properties, it is important that the MGpaper also has good mechanical strength in order for it to cope with the highdemands in the end packaging applications.

Microfibrillated cellulose (MFC) is known to be used as a strengthadditive or barrier additive when producing paper or paperboard products.However, MFC has a very high water binding capacity and it is thus verydifficult to reduce the water content of a slurry comprising microfibrillatedcellulose and the dewatering demand for a product comprising high amountsof MFC is very high. Thus, it is difficult to dewater a product comprising highamounts of MFC without deteriorating the mechanical or barrier properties ofthe product.

During production of machine glazed paper it is important that therunnability of the paper is improved. By adding barrier or strength additives tothe paper there is a risk with lifting or blistering of the web during drying.

There is thus a need for a new method to produce an improved MGpaper having good strength and barrier properties in an efficient way.

Summarylt is an object of the present invention to provide a method for producing a machine glazed paper comprising microfibrillated cellulose in anefficient way without negatively affecting the strength and barrier properties ofthe paper, which method further eliminates or alleviates at least some of thedisadvantages of the prior art methods.

The invention is defined by the appended independent claims.Embodiments are set forth in the appended dependent claims and in thefollowing description.

The present invention relates to a method for producing a machineglazed paper comprising microfibrillated cellulose, wherein the methodcomprises the steps of: providing a suspension comprising between 0.1 wt-%to 50 wt-% of microfibrillated cellulose based on total dry weight, forming afibrous web of said suspension on a wire wherein said web has a dry contentof 1-25% by weight, dewatering the fibrous web in at least one dewateringunit, glazing at least one side of the dewatered fibrous web in a glazing unit toform the machine glazed paper. lt has been found that it is possible produce a machine glazed paperwith good strength and barrier properties by the use of microfibrillatedcellulose. MG paper is a quite high density paper so it was surprisingly foundpossible to add quite high amounts of MFC to the suspension and still be ableto produce a MG paper having good strength and barrier properties at a highproduction speed, i.e. the combination of the dewatering unit and the glazingunit made it possible to still dewater and dry the fibrous web in an efficientway.

The dewatering unit is preferably a shoe press, a belt press or similarextended nip pressing equipment with a nip length of at least 150mm. lt wasfound that the use of a shoe press, belt press or similar extended nip pressingequipment and a glazing unit made it possible to improve the dewatering ofthe web without destroying the barrier properties of the fibrous web.

The glazing unit is preferably a Yankee cylinder. lt was found that theuse of a Yankee cylinder as a glazing unit and the dewatering unit made itpossible to both dry and provide the at least one surface of the fibrous webwith a glazed surface in an efficient way. 3 The fibrous web may be calendered in a calender after beingconducted through the glazing unit. Any know calender can be used. lt ispossible to calender one or both sides of the machine glazed paper.

The fibrous web preferably has a dry content between 25-45 wt-% afterbeing conducted through the at least one dewatering unit. The fibrous webpreferably has a dry content above 35 wt-% before being treated in theglazing unit, preferably above 45 wt-%, The dry content of the fibrous webbefore being treated in the glazing unit is preferably below 85 wt-%, morepreferably between 35-85 wt-% or even more preferred between 45-85 wt-%.By using the mentioned solid contents of the fibrous web before being treatedin the dewatering unit and the glazing unit, a machine glazed paper withimproved strength, good barrier properties and be produced in an efficientway.

The suspension may also comprise a hydrophobizing chemical such asAKD, ASA or rosin size in an amount of 0.1-10 kg/ton, preferably 0.1-5 kg/tonand more preferably 0.2-2 kg/ton based on dry weight. By adding anhydrophobizing chemical to the suspension as an internal sizing agent thebarrier properties of the machine glazed paper is improved. lt was also foundthat the combination of MFC and hydrophobizing chemical improved theadhesion of the web to the glazing unit which improved the runnability of theprocess The fibrous web may comprise more than one layer comprisingmicrofibrillated cellulose. ln this way a multiply paper comprising more thanone layers comprising microfibrillated cellulose is formed. The fibrous webcomprising more than one layers comprising microfibrillated cellulose can beformed by subjecting at least two suspensions comprising microfibrillatedcellulose to a wire. The at least two suspensions may be added to the wireeither in a multiply headbox or by the use of two different headboxes. The atleast two suspensions whereof at least one of the suspensions comprisesmicrofibrillated cellulose are applied to said wire so that the first suspensionapplied onto the wire, i.e. in direct contact with said wire and the othersuspension is applied onto the applied first suspension. ln this way a multiplyfibrous web is formed. lt may also be possible to attach two or more fibrouswebs together after formation on a wire, to form a multiply paper product, i.e.a first fibrous web is formed on a first wire from a first headbox and a secondfibrous web is formed on a wire support from a second headbox. The first andsecond fibrous webs are thereafter attached to each other to form a multiply fibrous web. Consequently, it is also possible to produce a multiply fibrousweb by using two, three or more headboxes and wires and then attach thefibrous webs produced to each other and conduct the multiply fibrous webcomprising more than one fibrous web through a dewatering unit and aglazing unit to produce the machine glazed paper. lt might be preferred toproduce a three layer machine glazed paper where only the suspensionforming the midply of the MG paper comprises MFC. ln this way the amountof MFC in the midply can be increased, which will improve the strength andbarrier properties of the paper without the drawback with release or notenough adhesion to the surface of the glaze unit.

The produced machine glazed paper is preferably coated on at leastone side with a coating composition. The coating composition preferablycomprises water-soluble polymers such as cellulose, starch, nanocellulose,cellulose derivatives, such as carboxymethyl cellulose, starch derivatives,polyvinyl alcohol or polyvinyl alcohol derivatives, or combinations thereof. Thesaid suspension might further comprise performance or functional chemicalssuch as cross-linkers, nanofillers or softening agents. lt is preferred that thecoating is applied to the glazed surface of the MG paper. The coatingcomposition will further improve the barrier properties of the paper. lt wassurprisingly found that the addition of MFC to the paper improved the coatingproperties of the paper, i.e. the coverage of the coating on the surface of thepaper is strongly improved. One theory is that the density of the glazedsurface is increased meaning that the coating "stays" on the surface of thepaper and it is possible to reduce the coating amount and still be able toachieve an even coating on the surface. lt is preferred that the coating isapplied in amount of0.1-5 gsm, preferably between 0.2-4 gsm and even morepreferred between 0.3-3 gsm.

The present invention further relates to a machine glazed paperproduced according to the method above comprising 0.1-50 wt-% ofmicrofibrillated cellulose. The machine glazed paper preferably has anOxygen Transmission Rate (OTR) value (23°C, 50% RH) below 200cc/m2/24h according to ASTM D-3985, a grammage between 25-160 gsm, aGurley Hill value of at least 25000 s/100ml, and more preferably at least40 000 s/100ml, as measured according to standard ISO 5636/6, at least oneglazed surface with a surface roughness PPS value below 5um according toISO 8791 -4, preferably below Zum (before addition of any eventual coating), aKIT value of at least 6, more preferably above 8 and a Scott Bond of value above 1500 J/m2, more preferably higher than 1600 J/m2 and most preferablyhigher than 1800 J/m2 measured according to TAPPI UM-403 on a 60 gsmpapen Detailed descriptionWith the present invention it was found possible to produce a machine glazed paper comprising MFC having improved strength, good barrierproperties still at a high production speed. Since the dewatering process oftenis the most cha||enging process step for the production of a paper productcomprising high amounts of MFC, the production speed of the entire productline can also be improved by improving the dewatering process. lt wassurprisingly found that the combination of a dewatering unit, preferably byusing an extended nip pressing equipment such as a shoe press in followedby a glazing unit, preferably a Yankee Cylinder made it possible to produce amachine glazed paper comprising microfibrillated cellulose in a good andefficient way.

The suspension comprises between 0.1 wt-% to 50 wt-% ofmicrofibrillated cellulose based on total dry weight, preferably between 2-40wt-% or even more preferred between 5-30 wt-% of MFC based on the totaldry weight. Besides MFC the suspension also comprises cellulosic fibers,preferably chemical pulp based fibers, such as kraft pulp fibers. Thesuspension may also comprise mechanical pulp fibers orchemothermomechanical (CTMP) pulp fibers. The suspension preferablycomprises 50-99.9- wt-% of cellulosic fibers based on the total dry weight,preferably between 60-98 wt-% or even more preferred between 70-95-wt-%.The fibers may be hardwood or softwood fibers. The cellulosic fibers in thesuspension, i.e. both the "normal" fibers and the MFC, may be bleached toproduce a white paper product or unbleached to produce a brown paperproduct.

The microfibrillated cellulose of the suspension preferably has aSchopper-Riegler (SR) value above 80, preferably above 90, even morepreferably above 95, preferably between 90-100 or even more preferredbetween 95-100 as determined by standard ISO 5267-1. Consequently, thesuspension preferably comprises a fine grade MFC quality which normally isvery difficult to dewater.

The suspension may also comprise a hydrophobizing chemical such asAKD, ASA or rosin size in an amount of 0.1-10 kg/ton, preferably 0.1-5 kg/ton and more preferably 0.2-2 kg/ton based on dry weight. By adding anhydrophobizing chemical to the suspension as an internal sizing agent thebarrier properties of the machine glazed paper is improved. lt was found thatthe combination of MFC and hydrophobizing chemical improved the adhesionof the web to the glazing unit which then improved the runnability of theprocess The suspension may also comprise additives such as native starch orstarch derivatives, cellulose derivatives such as sodium carboxymethylcellulose, a fi||er, retention and/or drainage chemicals, f|occu|ation additives,deflocculating additives, dry strength additives, softeners, cross-linking aids,dyes and colorants, wet strength resins, fixatives, de-foaming aids, microbeand s|ime control aids, or mixtures thereof.

The wire is preferably a wire in a paper or paperboard machine and thedewatering and production of the machine glazed paper is preferably done ina paper machine .A paper machine (or paper-making machine) is anindustrial machine which is used in the pulp and paper industry to createpaper in large quantities at high speed. Modern paper-making machines aretypically based on the principles of the Fourdrinier Machine, which uses amoving woven mesh, a "wire", to create a continuous web by filtering out thefibers held in a pulp suspension and producing a continuously moving wetweb of fiber. This wet web is dried in the machine to produce a strong paperweb.

A fibrous web of said suspension is formed on a wire wherein said webhas a dry content of 1-25% by weight. The fibrous web is thereafter furtherdewatered or drained on the wire by any known method. The furtherdewatering typically comprises pressing the web to squeeze out as muchwater as possible. The further dewatering may for example include passingthe formed multilayer web through a press section of a paper machine, wherethe web passes between large rolls loaded under high pressure to squeezeout as much water as possible. The removed water is typically received by afabric or felt. The fibrous web is thereafter conduced trough at least onedewatering unit. The fibrous web preferably has a dry content between 25-45wt-% after being conducted through the at least one dewatering unit. Thefibrous web being dewatered in the dewatering unit is thereafter conductedthrough a glazing unit. lt is preferred that the fibrous web has a dry contentabove 35 wt-% before being treated in the glazing unit, preferably above 45wt-%. The dry content of the fibrous web before being treated in the glazing 7 unit is preferably below 85 wt-%, more preferably between 35-85 wt-% oreven more preferred between 45-85 wt-%. By using the mentioned solidcontents of the fibrous web before being treated in the dewatering unit andthe glazing unit, a machine glazed paper with improved strength, good barrierproperties and be produced in an efficient way.

The dewatering unit is preferably a shoe press, a belt press or similarextended nip pressing equipment with a nip length of at least 150mm. lt wasfound that the use of a shoe press, belt press or similar extended nip pressingequipment made it possible to improve the dewatering of the web withoutincreasing the risk for wet blistering of the web and destroying the barrierproperties of the fibrous web.

The extended nip pressing equipment preferably has a nip length of atleast 150 mm, preferably at least 200 mm, preferably between 150-350 mm,and even more preferred between 200 and 300 mm.

The linear load in expended nip pressing equipment is preferablybetween 250-1500 kN/m, i.e. this is the maximum linear load to be used in theequipment, e.g. the shoe press. lt is preferred that the linear load used ischanged during the treatment of the fibrous web. By gradually or stepwiseincreasing the linear load in the extended nip pressing equipment, thedewatering of the web is improved, i.e. a web with a higher dry content can beproduced without destroying the barrier properties. lt is also possible that thelinear load is increased at a pulse during treatment in the nip, i.e. the linearload is increased at least one time in at least one pulse during treatment ofthe fibrous web in the shoe pres. This can be repeated during treatment in theextended nip pressing equipment. lf more than one extended nip pressingequipment, e.g. shoe presses, is used it is possible to use the same linearload profile in both equipment. However, it is often preferred to use differentlinear load profiles to design the linear load profile in such a way that thedewatering is improved without deteriorating the barrier properties of thedewatered web.

With shoe press is meant an extended nip pressing equipmentcomprising a shoe press nip. Any known shoe press can be used. The shoepress nip can either be formed by using a shoe and a roll or by using a largediameter soft roll and a roll. The roll preferably has a synthetic belt but it canalso have a metal belt. The large diameter soft roll can have a diameter of1.5-2 meters.

The position of the shoe in relation to the fibrous web can be changedby changing the tilt angle of the shoe press. The tilt angle of the at least oneshoe press is preferably between 7-24 degrees. The tilt angle affects the peaklinear load and is a way to adjust the linear load to improve the dewateringefficiency of the web.

The nip time is preferably at least 30 ms. Depending on the nip lengthand the production speed the time in which the fibrous material is subjected tothe pressure in the shoe press varies.

With belt press is meant an extended nip pressing equipmentcomprising a belt. Any known belt presses can be used. lt may be preferred to use at least two extended nip pressingequipment, preferably at least two shoe presses, and that the two extendednip pressing equipment are being located after each other. The fibrous web isthen first conducted through a first shoe press and then through the secondshoe press. ln this way it was found possible to even further improve thedewatering of the fibrous web and still be able to produce a paper with goodbarrier properties. The nip pressure used in the first shoe press is preferablylower than the nip pressure used in the second shoe press. The at least twoshoe presses are preferably located at different sides of said fibrous web. lnthis way it is possible to dewater the web from both directions through thefibrous web. When more than one shoe press is used is it preferred that thetotal nip length, i.e. the sum of the nip lengths of each shoe press, is above350 mm, preferably above 400 mm and even more preferred above 450 mm.The geometric design of the at least two shoe presses is preferably different,e.g. one shoe press can have a concave design and one shoe press canhave a convex design.

The glazing unit is preferably a Yankee cylinder. lt was found that theuse of a Yankee cylinder as a glazing unit made it possible to both dry andprovide the at least one surface of the fibrous web with a glazed surface.Yankee Cylinders are normally used for drying tissue papers that is a veryporous material. The use of Yankee Cylinders and how the drying affectspaper is well described by Walker, in the article "High temperature YankeeHoods Save Energy and Improve Quality, P&P, July 2007. When using aYankee Cylinder for drying products, the liquid in the products flows throughthe product towards the Yankee cylinder, i.e. towards the heat and the steamthat is formed during the drying. The liquid of the product in our case also 9 comprises microfibrils which leads to that an increased concentration ofmicrofibrils is achieved on the smoothened and glazed surface of the paper.

The temperature of the glazing unit is preferably above 100°C,preferably between 110-190°C. The first side of the fibrous web be in directcontact with the glazing unit, e.g. in direct contact with the surface of theYankee cylinder. ln order to control the adhesion of the fibrous web to the glazing unit,e.g. Yankee cylinder, it may be preferred to add adhesion control additives tothe surface of the glazing unit. This has been showed to be more importantwhen microfibrillated cellulose is used since the microfibrillated cellulose inthe fibrous web tend to make the fibrous web too tense which causes lifting orblistering of the web from the surface of the glazing unit. The adhesion controladditives will provide sufficient adhesion of the web to the surface of theglazing unit. Suitable adhesion control additives may be water-soluble orpartly water-soluble polymers such as polyvinyl alcohol (PVOH), polyamide-amine derivate, polyethylene imine, polyacrylamide and/or polyacrylamidederivate. The degree of hydrolysis of the PVOH used is preferably less than99%, even more preferred less than 98%. lt is also possible to use modifiedpolymers, such as modified PVOH, preferably ethylene, carboxylated,cationized or siliconized PVOH. The adhesion control additive may alsocomprise nanoparticles, such as nanoclay and/or nanocellulose. Theadhesion control additive may also comprise between 0.5-20 wt-% ofnanoparticles based on total dry weight. The amount of adhesion controladditive to the surface of the glazing unit is preferably between 0.1-10 gsm.The adhesion control additive is preferably added to the surface of the glazingunit by spraying. The adhesion control additive is preferably added to thesurface of the glazing unit as a solution or as a foam.

The fibrous web may be calendered in at least one calender after beingconducted through the glazing unit. Any know calender can be used, such asmachine calender, multi-nip calender, soft-nip calender, belt calender. lt maybe preferred to use a shoe calender or any other extended nip calender. lt ispossible to calender one or both sides of the machine glazed paper. Thetreatment in the calender is preferably done in-line.

The fibrous web may be treated in a de-curling unit after beingcalendered. ln this way it is possible to even further reduce the curlingtendency of the paper.

The fibrous web preferably comprises more than one layer ofcomprising microfibrillated cellulose. ln this way a multiply paper comprisingmore than one layer comprising microfibrillated cellulose is formed. Thefibrous web comprising more than one layer comprising microfibrillatedcellulose can be formed by subjecting at least two suspensions comprisingmicrofibrillated cellulose to a wire. The at least two suspensions may beadded to the wire either in a multiply headbox or by the use of two differentheadboxes. lt may also be possible to use other application methods as well,e.g. spray or curtain such as a flexJet headbox to create the multilayeredfibrous web. The at least two suspensions comprising microfibrillatedcellulose is applied to said wire so that the first suspension applied onto thewire, i.e. in direct contact with said wire and the other suspension is appliedonto the applied first suspension. ln this way a multiply fibrous web is formed.lt may also be possible to attach two or more fibrous webs together afterformation on a wire, to form a multiply paper product, i.e. a first fibrous web isformed on a first wire from a first headbox and a second fibrous web isformed on a wire support from a second headbox. The first and secondfibrous webs are thereafter attached to each other to form a multiply fibrousweb. The at least two suspensions comprising microfibrillated cellulose maycomprise the same type, amount, consistency etc of microfibrillated celluloseor different types, amounts, consistencies etc of the at least two suspensionmay be used. The multilayer fibrous web may comprise two, three, four, fiveor more layers. Consequently, it is also possible to produce a multiply fibrousweb by using two, three or more headboxes and wires and then attach thefibrous webs produced to each other and conduct the multiply fibrous webcomprising more than one fibrous web through a dewatering unit and aglazing unit to produce the machine glazed paper.

The produced machine glazed paper is preferably coated on at leastone side with a coating composition. The coating composition preferablycomprises starch, carboxymethyl cellulose and/or microfibrillated cellulose. ltis preferred that the coating is applied to the glazed surface of the MG paper.The coating composition will further improve the barrier properties of thepaper. lt was surprisingly found that the addition of MFC to the paperimproved the coating properties of the paper, i.e. the coverage of the coatingon the surface of the paper is strongly improved. One theory is that thedensity of the glazed surface is increased meaning that the coating "stays" onthe surface of the paper and it is possible to reduce the coating amount and 11 still be able to achieve a full coating coverage on the surface. lt is preferredthat the coating is applied in amount of 0.1-5 gsm, preferably between 0.2-4gsm and even more preferred between 0.3-3 gsm. Any known coatingtechniques may be used to apply the coating composition to the surface ofthe paper.

The present invention further relates to a MG paper producedaccording to the method described herein. The MG paper comprises 0.1-50wt-% of microfibrillated cellulose, preferably between 2-40 wt-% or even morepreferred between 5-30 wt-%.

The machine glazed paper preferably has an Oxygen TransmissionRate (OTR) value (23°C, 50% RH) below 200 cc/m2/24h according to ASTMD-3985, preferably below 150 cc/m2/24h and even more preferred below 100cc/m2/24h.

The MG paper preferably has a grammage between 25-160 gsm,preferably between 30-140 gsm or even more preferred between 40-130 gsm.

The MG paper preferably has a Gurley Hill value of at least 25 000s/100ml, preferably at least 40 000 s/100ml, and more preferably at least60 000 s/100ml, as measured according to standard ISO 5636/6.

The MG paper preferably has at least one glazed surface with asurface roughness PPS value below 5um according to ISO 8791-4, preferablybelow 2um (before addition of any eventual coating).

The MG paper preferably has a Scott Bond value above 1500 J/m2,more preferably above 1600 J/m2 and most preferably above 1800 J/m2measured according to TAPPI UM-403 on a 60 gsm paper. Consequently, theMG paper produced has very high strength.

The MG paper will typically exhibit good resistance to grease and oil.Grease resistance of the paper is evaluated by the KIT-test according tostandard ISO 16532-2. The test uses a series of mixtures of castor oil,toluene and heptane. As the ratio of oil to solvent is decreased, the viscosityand surface tension also decrease, making successive mixtures more difficultto withstand. The performance is rated by the highest numbered solutionwhich does not darken the sheet after 15 seconds. The highest numberedsolution (the most aggressive) that remains on the surface of the paperwithout causing failure is reported as the "kit rating" (maximum 12). In someembodiments, the KIT value of the MG paper is at least 6, preferably at least8, and even more preferred at least 10, as measured according to standardISO 16532-2. 12 The MG paper preferably has high repulpability. ln some embodiments,the multilayer MG paper exhibits less than 30 %, preferably less than 20 %,and more preferably less than 10 % reject, when tested as a category llmaterial according to the PTS-RH 021/97 test method.

Microfibrillated cellulose (MFC) shall in the context of the patentapplication mean a nano scale cellulose particle fiber or fibril with at least onedimension less than 1000 nm. MFC comprises partly or totally fibrillatedcellulose or lignocellulose fibers. The liberated fibrils have a diameter lessthan 1000 nm, whereas the actual fibril diameter or particle size distributionand/or aspect ratio (length/width) depends on the source and themanufacturing methods. The smallest fibril is called elementary fibril and hasa diameter of approximately 2-4 nm (see e.g. Chinga-Carrasco, G., Ce/lu/osefibres, nanofibrils and microfibrils,: The morpho/ogica/ sequence of MFCcomponents from a plant physiology and fibre technology point of view,Nanoscale research letters 2011, 6:417), while it is common that theaggregated form of the elementary fibrils, also defined as microfibril (Fengel,D., U/trastructura/ behavior of cell wall polysaccharides, Tappi J., March 1970,Vo/53, No. 3.), is the main product that is obtained when making MFC e.g. byusing an extended refining process or pressure-drop disintegrationprocess. Depending on the source and the manufacturing process, the lengthof the fibrils can vary from around 1 to more than 10 micrometers. A coarseMFC grade might contain a substantial fraction of fibrillated fibers, i.e.protruding fibrils from the tracheid (cellulose fiber), and with a certain amountof fibrils liberated from the tracheid (cellulose fiber).

There are different acronyms for MFC such as cellulose microfibrils,fibrillated cellulose, nanocellulose, nanofibrillated cellulose, fibril aggregates,nanoscale cellulose fibrils, cellulose nanofibers, cellulose nanofibrils, cellulosemicrofibers, cellulose fibrils, microfibrillar cellulose, microfibril aggregrates andcellulose microfibril aggregates. MFC can also be characterized by variousphysical or physical-chemical properties such as large surface area or itsability to form a gel-like material at low solids (1-5 wt-%) when dispersed inwater. The cellulose fiber is preferably fibrillated to such an extent that thefinal specific surface area of the formed MFC is from about 1 to about 200m2/g, or more preferably 50-200 m2/g when determined for a freeze-driedmaterial with the BET method.

Various methods exist to make MFC, such as single or multiple passrefining, pre-hydrolysis followed by refining or high shear disintegration or 13 Iiberation of fibrils. One or several pre-treatment step is usually required inorder to make MFC manufacturing both energy efficient and sustainable. Thecellulose fibers of the pulp to be supplied may thus be pre-treatedenzymatically or chemically, for example to hydrolyse or swell fiber or reducethe quantity of hemicellulose or lignin. The cellulose fibers may be chemicallymodified before fibrillation, wherein the cellulose molecules contain functionalgroups other (or more) than found in the original cellulose. Such groupsinclude, among others, carboxymethyl (CMC), aldehyde and/or carboxylgroups (cellulose obtained by N-oxyl mediated oxidation, for example"TEMPO"), or quaternary ammonium (cationic cellulose). After being modifiedor oxidized in one of the above-described methods, it is easier to disintegratethe fibers into MFC or nanofibrillar size or NFC.

The nanofibrillar cellulose may contain some hemicelluloses; theamount is dependent on the plant source. Mechanical disintegration of thepre-treated fibers, e.g. hydrolysed, pre-swelled, or oxidized cellulose rawmaterial is carried out with suitable equipment such as a refiner, grinder,homogenizer, colloider, friction grinder, ultrasound sonicator, fluidizer such asmicrofluidizer, macrofluidizer or fluidizer-type homogenizer. Depending on theMFC manufacturing method, the product might also contain fines, ornanocrystalline cellulose or e.g. other chemicals present in wood fibers or inpapermaking process. The product might also contain various amounts ofmicron size fiber particles that have not been efficiently fibrillated.

MFC is produced from wood cellulose fibers, both from hardwood or softwoodfibers. lt can also be made from microbial sources, agricultural fibers such aswheat straw pulp, bamboo, bagasse, or other non-wood fiber sources. lt ispreferably made from pulp including pulp from virgin fiber, e.g. mechanical,chemical and/or thermomechanical pulps. lt can also be made from broke orrecycled paper. ln view of the above detailed description of the present invention, othermodifications and variations will become apparent to those skilled in the art.However, it should be apparent that such other modifications and variationsmay be affected without departing from the spirit and scope of the invention.

Claims (18)

1. A method for producing a machine glazed paper comprisingmicrofibrillated cellulose, wherein the method comprises the steps of: - providing a suspension comprising between 0.1 wt-% to 50 wt-%of microfibrillated cellulose based on total dry weight, - forming a fibrous web of said suspension on a wire wherein saidweb has a dry content of 1-25% by weight, - dewatering the fibrous web in at least one dewatering unit, - glazing at least one side of the dewatered fibrous web in aglazing unit to form the machine glazed paper.

2. The method as claimed in claim 1, wherein the dewatering unitis a shoe press, belt press or similar extended nip press unit with a niplength of at least 150mm.

3. The method according to any of the preceding claims whereinthe glazing unit is a Yankee cylinder.

4. The method according to any of the preceding claims whereinthe fibrous web is calendered in at least one calender after beingconducted through the glazing unit.

5. The method according to any of the preceding claims whereinthe fibrous web has a dry content between 25-45 wt-% after beingconducted through the at least one dewatering unit.

6. The method according to any of the preceding claims whereinthe fibrous web has a dry content above 35 wt-% before being treatedin the glazing unit.

7. The method according to any of the preceding claims whereinsuspension further comprises a hydrophobizing chemical in an amountof 0.1-10 kg/ton, preferably 0.1-5 kg/ton and more preferably 0.2-2kg/ton based on dry weight.

8. The method according to claim 7 wherein the hydrophobizingchemical is AKD, ASA or rosin size.

9. The method according to any of the preceding claims whereinthe fibrous web comprises more than one layer comprisingmicrofibrillated cellulose.

10. The method according to any of the preceding claims whereinthe produced machine glazed paper is coated on at least one side witha coating composition.

11. The method according to claim 10 wherein the coatingcomposition comprises water-soluble polymers such as cellulose,starch, nanocellulose, cellulose derivatives, such as carboxymethylcellulose, starch derivatives, polyvinyl alcohol or polyvinyl alcoholderivatives or combinations thereof.

12. A machine glazed paper produced according to any of theclaims 1-11 wherein the machine glazed paper comprises 0.1-50 wt-%of microfibrillated cellulose.

13. The machine glazed paper according to claim 12 wherein themachine glazed paper has a grammage between 25-160 gsm.

14. The machine glazed paper according to claims 12-13 whereinthe machine glazed paper has an Oxygen Transmission Rate (OTR)value (23°C, 50% RH) below 200 cc/m2/24h according to ASTM D-3985.

15. The machine glazed paper according to claims 12-14 whereinthe machine glazed paper has Gurley Hill value of at least 25000s/100ml, and more preferably at least 40 000 s/100ml, as measuredaccording to standard ISO 5636/6.

16. The machine glazed paper according to claims 12-15 whereinthe machine glazed paper has at least one glazed surface with a 16 surface roughness PPS value below 5um according to ISO 8791-4,preferably below Zum.

17. The machine glazed paper according to claims 12-15 whereinthe machine glazed paper has a Scott Bond value above 1500 J/m2measured according to TAPPI UM-403 on a 60 gsm paper.

18. The machine glazed paper according to claims 12-16 whereinthe machine glazed paper has a KIT value of at least 6 measuredaccording to standard ISO 16532-2.